Security researchers at the Where 2.0 conference have discovered that your iPhone keeps track of everywhere you go. All the time. The Guardian has the story.

This info is not transmitted to Apple but it stays on your phone and is synced to your computer and new devices. This means that someone who stole your computer or your phone could access this file and see everywhere you've been.

This is obviously a huge privacy violation and it's not clear why Apple would do such a thing. The researchers haven't been able to find a similar "feature" on Android.